This invention generally relates to laser diode arrays, and more specifically to such arrays sometimes referred to as multi-stripe arrays that produce multiple laser beams with adjacent beams being 180.degree. out of phase.
This phase relationship produces a double lobed beam pattern in the far field of the laser array which for some applications is undesirable. In accordance with the present invention, this out of phase relationship of the beams is corrected so that the beams are in phase and a single lobed beam pattern is produced in the far field.
Methods for generating single lobed far field patterns from a laser diode phased array have been reported in literature. See for example, J. Katz, S. Margalit, and A. Yariv, "Diffraction Coupled Phase-Locked Semiconductor Laser Array," Appl. Phys. Lett., Vol. 42, No. 7, 1983, pp. 554-556; J. Katz, Invention report prepared by J. T. English, "Semiconductor Laser Phased Array," NASA Tech. Brief, Vol. 8, No. 3, Item #6 from JPL Invention Report NPO-15963/5417; J. E. Epler, N. Holonyak, Jr., R. D. Burnham, T. L. Paoli, and W. Streifer, "Far Field Supermode Patterns of a Multiple-Stripe Quantum Well Heterostructure Laser Operated (.about.7330 A, 300.kappa.) in an External Grating Cavity," App. Phys. Lett., Vol. 45, 1984, pp. 406-408; and D. E. Ackley, D. Botez and B. Bogner, "Phase-Locked Injection Laser Arrays with Integrated Phase Shifters," RCA Review, Vol. 44, 1983, pp. 625-633. In the last of these articles the author describes a concept for achieving zero phase shifts between all of the array elements by coating selected ones of the diode array facets so that the beams are in phase and a single lobed far field pattern is achieved. It is not known to applicant whether that concept has been successfully implemented. By the present invention, a single lobed far field pattern is achieved by the use of a separate phase corrector plate positioned in the near field at the image of the array facet, such that the beams after passing through the phase corrector plate are substantially in phase. This technique is easy and inexpensive to implement.
Generally in accordance with the invention there is provided a phase corrector plate having multiple sets of regions. The phase plate is positioned in the near field of the beam pattern such that the beams of one phase from the laser diode array pass through one set of regions, and the beams of the other phase from the laser diode array pass through the other set of regions. One set of regions has means, such as a coating, to change the phase of the beams passing therethrough so as to be substantially in phase with the beams that pass through the other set of regions. As a result, all of the beams passing through the phase plate are in phase to produce a single lobed beam pattern in the far field.